1. Field of The Invention
The present invention relates to a diagnosing apparatus and method for detecting an occurrence of failure in a rear road wheel steering angle sensor applicable to a motor driven four road wheel power steering system which controls a rear road wheel steering angle by means of an electric motor.
2. Description of Background Art
A Japanese Patent Application First Publication No. Showa 63-82875 published on Apr. 13, 1988 exemplifies a previously proposed motor driven power assisted steering system in which an occurrence in failure of a steering angle sensor used in a power assisted control for the steering of front road wheels and whose output signal is used as a positioning feedback information.
In the above-identified Japanese Patent Application Publication, two of main and sub steering angle sensors are used, a change rate of each output signal of the main and sub steering angle sensors is calculated, and a determination of occurrence of failure in either of the two steering angle sensors which indicates a minimum change rate when a difference occurs in the change rate between output signals of the respective steering angle sensors. In addition, when the output signal of either of the two steering angle sensors exceeds a predetermined range, the determination of occurrence of failure in either of the steering angle sensors whose output signal exceeds the predetermined range is carried out.
However, since the steering angle sensors are such a duplex system configuration that one of the two steering angle sensors is used as the main steering angle sensor to perform a servo control for the electric motor and the other steering angle sensor is used as the sub steering angle sensor to monitor the occurrence of failure, the duplex system configuration of the above-described power assisted steering control system worsens a cost effectiveness.